Influential theories concerning personality argue that many impulsive individuals show physiological underarousal at rest. This interoceptive state is proposed to be egodystonic, motivating impulsive maladaptive actions to enhance arousal. However, there is little empirical research on this matter. The current study tested the relationship between physiological markers of arousal, measures of interoceptive (in)sensitivity, and trait impulsivity in a nonclinical sample of young adults. Experiment 1 investigated whether individuals (N = 31) with high trait impulsivity show decreased resting measures of arousal (indexed from heart rate, heart rate variability, and sympathetic electrodermal activity). Experiment 2 assessed whether trait impulsivity is linked to interoceptive abilities (N = 60). Overall, our results do not provide any compelling support for the underarousal theory of impulsivity. However, impaired interoceptive (cardiac discrimination) accuracy predicted the degree of Barratt nonplanning impulsivity, such that individuals with a better ability to distinguish between internal (bodily) and external signals manifest lower levels of nonplanning trait impulsivity. These findings open an avenue for potential novel interventions aimed at improving planning abilities via better interoceptive discrimination. The current study explored the relationship between physiological markers of resting arousal, measures of interoceptive (in)sensitivity, and trait impulsivity. Overall, our results do not provide any compelling support for the underarousal theory of impulsivity, stating that impulsive individuals show physiological underarousal at rest. Instead, impaired interoceptive (cardiac discrimination) accuracy predicted the degree of Barratt nonplanning impulsivity, suggesting that lower levels of nonplanning trait impulsivity might be in part due to impaired discrimination between internal (heartbeats) and external (auditory tones) signals. These findings open an avenue for potential novel interventions aimed at improving planning abilities via better interoceptive discrimination.